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Thread: toilet vent size issue

  1. #1

    Default toilet vent size issue

    I posted this in remodeling - but it may be a better topic for this forum:

    I am remodeling a powder room on first floor of two story 1925 house. Used to have a pedestal and a toilet on one side and a shower opposite. shower sucked and was not needed, sink and toilet were crowded. Took out all three, and am putting in a large vanity/Lav where the sink and toilet used to be; and putting a new toilet where the shower was. The D and W part of the DWV are pretty easy. Venting, though . . . There is a 2" vent line (easy to use) for the new Lav. There is a nice, straight vertical 1.5" vent that goes up through the roof and down to the crawl space, not 2 horizontal feet from where the new closet flange/toilet will be. I REALLY don't want to change that to 2" -- Very easy to hook up as is, totally a bear, maybe not even possible to change it. So -- can i put in one single toilet with a 3 inch drain and a 1.5 inch vent ?! Is it code and, more importantly, will it work? Thanks in advance.

    Also - what alternatives? AAV in crawl space??

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    An AAV needs to be above the floor plane, you can't put it under the toilet. You could put in in the bathroom behind a removeable panel, but it must be accessable for repair and if there is a clog, above the place where it could leak.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3


    yeah i am not a fan of AAVs anyway.
    What about running about 10 feet "horizontal" in the crawl space over to the wall with the 2 inch line, going up and y-ing in to that vent (how far above sink does that have to be?) is it ok to have a long horizontal (properly inclined) before it goes up to the roof? thanks again.

  4. #4
    Forum Admin, Expert Plumber Terry's Avatar
    Join Date
    Aug 2004
    Bothell, Washington
    Blog Entries


    The toilet can be vented within six feet.
    Most codes require at least a 2" vent for the toilet.
    Will 1.5" work? Yes.


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